The operation of a conventional blade coating unit is single-sided against a roll. This causes the paper composition to be non-symmetric which results in curling of the paper after drying. Since only one side of the paper is coated at a time both-sided coating requires two separate coating units which of course makes the apparatus complex and expensive and reduces its controllability during operation. A hole in the paper stains the roll which breaks the web as paper adheres to the roll during the next revolution. In a conventional blade coater, coating slip is applied and smoothed by a blade separately. Between these phases the paper web absorbs water and binding agents from the slip whereby it is difficult to regulate the composition of the coating slip, furthermore the paper web is wetted, it swells and its quality thus decreases.
A Billblade coater (Deutsches Papierwirtschaft 1981/3, pages 162-164) provides a method for two-sides coating, which eliminates the problem of curling of the paper. One of the drawbacks of the Billblade method, however, is that the quality of the coating is not adequate for high-quality products. The quality of the coating is decreased as coating of the other side of the web is carried out against a roll and the coating film formed between the roll and the paper web must be split and the smoothness of the coating layer thus disturbed. In a Billblade coater the web is drawn downwards and as no dryers can be installed under the coater because of dirty conditions, the web must be drawn largely free into the first dryer. This results in instability of the web and difficulties in running. In the Billblade process the web is drawn through a vessel due to which the viscosity range is limited. High solids contents can not be used in the Billblade process. The geometry of the Billblade process in two-sided coating is non-symmetric due to the form of the gap and the rotation of the roll, which pumps slip to the roll side.
The drawbacks of the Billblade process have been suggested to be avoided, e.g. by the Twinblade by Inventing (Deutsche Papierwirtschaft 1981/3, pages 162 to 164). In the Twinblade the web runs upwards to a coating unit where excess amounts of slip is applied onto the web by special applicators after which the web continues its travel (apprx. 50 cm) upwards to a nip of opposing blades where excess slip is removed by a doctor. One of the disadvantages of the Twinblade is that the web is excessively wetted during the long contact of the web with the coating slip between the applicators and the blades. This weakens the web in particular in case of thin paper grades and results in swelling of the web and problems in running. As Twinblade employes two blades installed against each other the geometry is extremely sensitive to the position of the blades and even the slightest wearing of the blades or bending of the blade beams result in crucial changes in the geometry. Therefore the coating process is in practice very difficult to control. Furthermore, positioning the blades at a distance from the coating slip applicators affects the coating quality, blows cause holes in the slip layer.
To improve conventional one-sided coating, so-called short-dwell coaters have been developed (Das Papier 37, 1983, no. 7, pages 303 to 305), where the coating slip is fed into the blade gap. The paper and the slip are in contact with each other only a short time (&lt;10 cm) before the slip is smoothed. This has been found to improve the running properties of the web and controllability of the coating process.
A Finnish patent application, no. 803184, discloses a coater where the web runs in a vertical direction upwards from the nip of a short-dwell type coating unit and a roll. Coating slip is provided onto the roll side by a separate applicator as close to the roll as possible and excess slip is returned down towards the applicator.
This method, however, has some drawbacks, e.g. the long contact time of the paper and the slip on the roll side; most of the slip flowing in with the paper is returned to the roll side which creates turbulence; the roll pumps coating slip to the smoothing gap of the coater and the coating film is splitted on the roll side.
In addition to the above basic coater applications there are several variant modifications available, which are more or less strained by the handicaps discussed above.